There are many processes using a wide variety of adsorbents and apparatus for separating various components from multi-component gaseous streams. Some processes are directed to purifying a gaseous stream by removing undesirable components while others are directed to removing and purifying a desirable component, such as hydrogen, that may be present in a stream in relatively small quantities. For example, fuel gas streams often contain substantial quantities of non-combustible components, such as nitrogen and carbon dioxide, along with the desired combustible component, methane. One conventional method for removing nitrogen and carbon dioxide from such streams is solvent extraction employing a solvent, such as monoethanolamine. Another conventional gas separation method is heat and mass exchange (HME) which is described in Chemical Engineering Science 40 (1985) No. 11, pp 2019-2025. Other methods include pressure swing adsorption (PSA) and thermal swing adsorption (TSA). Such processes are employed for a variety of applications where it is desired to separate a gaseous mixture into a first stream and a second stream. For example, the first stream can be one enriched relative to the feed mixture with respect to one or more component gases of the feed and the second stream can be enriched relative to the feed mixture with respect to one or more other component gases of the mixture.
Processes of the PSA and TSA type are normally operated on a cyclic basis using a plurality of beds of an adsorbent material. The cycle typically involves the basic steps of:
(a) passing a feed gas, at a first pressure, through an adsorbent bed wherein the more readily adsorbed component, or components, of the feed gas are adsorbed on the adsorbent while the less readily adsorbed component, or components, pass through the bed to give a first product stream at a pressure of the feed gas by the pressure drop provided by the resistance to flow of gas through the bed; PA1 (b) desorbing the adsorbed gas from the adsorption bed by reducing the pressure and/or by increasing the temperature, e.g. by the passage of a heated regeneration gas therethrough; the desorbed gas thus gives a second product stream comprising the more readily adsorbed component, or components, of the feed gas; and PA1 (c) returning the adsorbent bed to the adsorption step (a).
Alternatively, a non-dynamic PSA cycle may be employed wherein, for example, a bed is pressurized with the feed gas, depressurized in two or more stages. In the initial depressurized stage or stages, the less readily adsorbed component, or components, are released and in a subsequent depressurization stage, or stages, the more readily adsorbed component, or components, are released.
As is well known in the art, various other steps can be included in the cycle, for example one or more pressure equalization steps, sweeping steps, rinsing steps, and/or purging steps can be used. The desorption can be effected in stages to give one or more streams at an intermediate pressure and/or temperature and containing components of intermediate adsorbability, as well as a stream containing the more readily adsorbed components. Examples of various PSA cycles are described in EP-A No. 183358.
The size of the adsorbent bed employed for any given separation depends on such things as the duration of the longest step in the cycle, e.g. the sorption or the desorption/regeneration step. The required volume of the sorbent bed in turn depends on the effectiveness of the sorbent which in turn depends, inter alia, on the ratio of the geometric surface area of the adsorbent to the volume of adsorbent present in the bed. Generally adsorbents have been employed in the form of pellets, e.g. small cylinders, or spheres. The geometric surface area of the sorbent can of course be increased by making the pellets smaller, but this has the disadvantage that, for a bed of given length to cross-sectional area ratio, the tendency of the bed to fluidization, and also the pressure drop across the bed, is increased. This is often undesirable from an economic point of view. The present invention employs a novel class of layered nanostructures having at least 5% crystallinity.
The separation of methane from gas mixtures, which also contains carbon dioxide, is disclosed in U.S. Pat. No. 4,077,779 which teaches a PSA system generally applicable to bulk separation of various gas mixtures, including the separation of carbon dioxide from its admixture with methane, in a six step cycle, wherein following selective adsorption of one of the components of the mixture, the adsorbent bed is rinsed with part of the adsorbed component at super atmospheric pressure. The pressure in the rinsed bed is lowered to an intermediate level to desorb the same, the withdrawn gas in this step being employed in the high pressure rinse step. The bed is next purged with an extraneous gas (air or inert), evacuated to remove the purge gas, followed by bringing the bed back to super atmospheric pressure for repetition of the cycle.
Also, U.S. Pat. No. 4,229,188 teaches a process for recovering hydrogen from a gas mixture containing hydrogen and normally liquid hydrocarbons. The feed stream is passed to a selective adsorption unit to initially separate the feed. The purge stream from the adsorption unit is substantially treated in a membrane separator to recover an additional amount of the desired component. Further, U.S. Pat. No. 4,238,204 discloses a process for recovering a light gas in both high purity and high yield from a gas mixture containing said light gas and other components. The gas mixture is initially directed to a selective adsorption unit which produces a high purity light gas and a purged gas containing at least a portion of the light gas. The purge gas from the adsorption unit is subsequently passed to a membrane permeator selectively permeable to the light gas in order to recover the permeated gas comprising light gas of improved purity.
U.S. Pat. No. 4,398,926 teaches a process for recovering hydrogen from a high pressure stream having a hydrogen content of up to about 90 mole %. The feed stream is passed to a separator containing a permeable membrane capable of selectively permeating hydrogen. The separator is used to achieve a bulk separation of the desired hydrogen component from impurities contained in the gas stream. The separated hydrogen is recovered at reduced pressure and passed to a pressure swing adsorption system adapted for operation at reduced pressure. Additionally, the off gas from the separator is recovered essentially at the higher pressure of the feed gas stream, and at least a portion of this stream is throttled to a lower pressure and passed to the pressure swing adsorption system as a co-feed gas in order to increase the recovery of the desired component.
Also, the production of hydrogen by the steam reforming of hydrocarbons is well known. The effluent from the reformer furnace is principally hydrogen, carbon monoxide, and carbon dioxide in proportions close to equilibrium amounts at the furnace temperature and pressure with a minor amount of methane. The effluent is conventionally introduced into a one- or two-stage shift reactor to form additional hydrogen and carbon dioxide. The shift reactor converts the carbon monoxide to carbon dioxide with the liberation of additional hydrogen by reaction at high temperature in the presence of steam. The combination of hydrogen steam reformer and shift converter is well known to those of ordinary skill in the art. There have been proposed a number of schemes for treating the effluent from the shift converter to recover hydrogen and carbon dioxide therefrom, some of which include PSA techniques. For example, U.S. Pat. No. 4,963,339 teaches a process for producing highly purified gaseous hydrogen and carbon dioxide from a steam reformer/shift converter by passing the effluent from the latter through a multi-bed hydrogen PSA unit followed by an uncoupled carbon dioxide PSA unit. The carbon dioxide PSA unit produces a hydrogen-rich stream which is recycled to the feed to the steam reformer. A carbon dioxide-rich recycle stream is recycled to the carbon dioxide PSA unit feed and a carbon dioxide-rich product stream is introduced under pressure to a liquifier. A waste stream from the liquifier is recycled to the carbon dioxide PSA unit feed. A portion of the carbon dioxide rich product stream is withdrawn from the compressor at a stage such that its pressure is higher than that of the carbon dioxide PSA unit and returned thereto as a co-current purge preceding bed regeneration to obtain product.
While a number of the prior art processes for separating components from gaseous streams have been commercially practiced, there is still a need in the art for improved and alternative sorbents for such processes.